1. Technical Field of the Invention
The invention relates specifically to a flux-filled plunging lance and procedures to repair and restore the operating efficiency and capacity of slag-clogged channel and pressure-pour furnaces used for melting and pouring gray, ductile and compacted graphite irons. The invention can be used for non-ferrous melting and pouring applications. The proprietary additives are included as part of the invention as well as the design of the plunging assembly.
2. Description of the Related Art
Induction furnaces are refractory lined vessels that utilize electrical current flowing through copper coils to create an electromagnetic or induction field on the inside and outside of the coil. When electrical current from the coils is passed through the metallic furnace charge, joule heating creates thermal energy that melts the charge. Any metallic charge or molten metal mass passing through this induction field will generate heat internally and will eventually melt or will rise in temperature in the case of molten metal. The magnetic currents that arise in the molten metal produce an intense stirring action, thus insuring a homogenous liquid.
During the melting process, insoluble non-metallics or metallic oxides are generated from oxidation products, dirt, sand and other impurities from the scrap, erosion and wear of the refractory lining, oxidized ferroalloys, and other various sources. These non-metallics remain in the liquid metal as an emulsified “slag” until such time as they increase in size and buoyancy, coalesce and float on the liquid metal where they can be removed. Almost without exception, these generated slags will normally deposit in the upper portion of the lining or on side crucible walls. These areas are at a much lower temperature than the center of the furnace walls. Insoluble metallic oxides and sulfides that remain suspended within the molten metal will eventually deposit in areas where there is an interruption in the mechanical flow of the molten metal in the induction field. This normally corresponds to the midway point of the active power coil, along the vertical sidewall on the refractory lining. Eventually the clogging or restriction of furnace capacity will render the furnace useless and will affect the melting electrical efficiency.
Coreless induction furnaces can melt a wide range of metals. Steels, which melt at 2875° F. and higher, the entire family of cast irons which include gray and ductile irons with melting ranges of 2,475 to 2,850° F., and non-ferrous metals such as copper-base alloys (melting range of 1975° F. and higher) and aluminum-base alloys (melting range of 1175° F. and higher), can all be melted in induction furnaces.
An important type of induction furnace is the channel furnace. The principal difference between the two furnace types is in the placement of the induction coil and the metal bath. In coreless furnaces, the induction coil completely surrounds the melt cavity. In a channel furnace, the induction coil is part of an inductor assembly that performs the function of a “superheater of molten metal”. The concept is that the coil will create an induction field around the outside of the coil, and a separate loop or channel is placed at the optimal distance within the strength of the induction field. As molten metal is passed through this channel, the temperature will increase. This “heated molten metal” will then exit the inductor channel and eventually be displaced into a much larger volume of molten metal which is referred to as the “uppercase” that contains the major portion of the metal bath. This heating process and dispensing of the “heated metal” is a continuous process so long as power is available to the induction coil. When the induction channel experiences a clog or restriction in the molten metal flow, the electrical efficiency of the inductor is affected immediately and lost production from the inability to melt metal will result. Channel furnaces are typically used for processing cast irons, copper-base and aluminum-base alloys; they are not used for the processing of steel and steel alloys.
A type of channel furnace is the vertical channel furnace; it is often viewed as a large bull ladle or crucible with an inductor attached to the bottom. Accumulations of slag over time will typically occur in the bottom inductor loop or throat area. When this happens, insufficient metal flow through the inductor loop hampers heat transfer and interferes with the melting operation.
Typically, inductor life may be as long as 18 months. However, if slag build up occurs, the useful life may be reduced to only a few months or in some extreme cases, a few weeks. It is very difficult to remove accumulations of slag from the inductor loop or throat area. Often, the furnace will have to be taken out of operation and a new inductor installed. Because of the design of the furnace, access to the inductor loop is limited because it is covered by molten metal. Draining the furnace may allow visual access to the inductor throat and loop area, but mechanically scraping the slag buildup, which is normally quite hard and tenaciously adheres to the loop wall, is extremely difficult.
Pressure pour furnaces are sealed holding furnaces normally blanketed with a nitrogen atmosphere and that also have an induction coil attached to the bottom of the furnace. Pressure pour furnaces are designed to hold liquid metal at a constant temperature for periods of 1 to 3 days. When the pressure pour furnace is pressurized, a stream of molten metal exits the vessel for mold filling. These furnaces are not designed to melt metal. Circulation of liquid metal through the inductor throat or loop provides the heating of liquid metal to keep a constant temperature in the furnace.
Slags and/or drosses from the above-mentioned electric melting methods, if not totally removed at the melting furnace, will be transferred to the metal pouring ladles and eventually into the finished casting. This build-up must constantly be removed and a significant amount of labor is usually expended in keeping melting and pouring furnaces clean. Insoluble clogging and build-up often occurs in the inductor channels and throat transition areas. When this happens, the inductor will have to be replaced, since it is extremely difficult to remove the restricted clogged condition. If this condition is left unaddressed and slag buildup is not removed, molten metal will breach the inductor refractory and molten metal will run out of the furnace inductor. This is true for ferrous cast iron and non-ferrous channel furnaces and such run-outs represent a serious safety hazard.
Some success in removing slag buildup in inductor loops with specially designed fluxes have been reported by some cast iron and copper-base foundries but treatments may take several days of continuous flux additions. However, when severe restrictions from buildup occur, the furnace must be drained and a new inductor installed.
The addition of a flux to any molten metal traditionally lowers the melting point of slag. This action prevents the slag from freezing on the refractory surfaces. The use of a flux will usually ensure floatation of the emulsion of oxides and reduce the melting point of the slag to below the coldest temperature encountered in the melting, treatment and handling system to minimize slag build-up.
The use of fluxes can affect three important physical characteristics of slags: melting point, viscosity and wetting ability. Generally, slag is required to remain liquid at temperatures likely to be encountered during melting, metal treatment, or metal handling. Slag is required to be fluid for ease of removal from the melting furnace, to promote good slagging reactions and to prevent build-up in channel furnace throats and loops as well as coreless furnace sidewalls. In electric furnaces and pressure pour furnaces, slags must have a high interfacial surface tension to prevent refractory attack and to facilitate slag remove from the surface of the molten metal.
Most fluxes contain alkaline elements, such as sodium or calcium or barium, along with halide salts. Alkali metal halide salts with the elements of sodium and potassium are also commonly used in fluxes. Over the years, almost every conceivable combination of these compounds has been formulated into fluxes. Most commercial fluxes have been developed for the steel industry and for copper and aluminum melting applications. Very few fluxes have been developed for the electric melting segment of the cast iron industry, and those that have been developed, have been aimed at supplemental flux additions for cupola melting. Part of the reason is that silica-based refractory linings, because of low cost, are the preferred refractory of gray and ductile iron foundries. Fluxes that have been developed for the steel industry, where much higher quality refractories are used, will vigorously attack silica linings, hence, relatively few fluxes have been developed for this segment of the casting industry.
Fluorspar, a calcium fluoride mineral (CaF2), is a powerful fluxing agent that is commonly used in various proportions along with limestone and other metal halides or salts to improve slag fluidity in foundry cupola melting. Fluorspar, while effective, has certain serious disadvantages. Specifically, fluorspar is a very aggressive flux and works extremely well in integrated steel mill applications as well as cupola operations where non-silica refractory linings are normally employed. However, overzealous additions of fluorspar or fluorspar containing fluxes to electric melting furnaces will result in severe lining erosion.
From the foregoing it will be appreciated that serious shortcomings exist in cast iron and non-ferrous melting where channel furnaces or pressure pour vessels are employed. Elimination of inductor clogging and restrictions from slag buildup continues to be a troublesome problem and current methods to solve these problems usually involve taking the furnace off-line and replacing the clogged or restricted inductor.